Silicon single crystals fabricated by Czochralski Method (CZ method) contain a crystalline defect, BMD (Bulk Micro Defect). The BMD has a gettering ability that captures contaminant elements such as heavy metals within wafers to enhance properties of substrates. It is known that the BMD in the silicon single crystal goes up by carbon doping at the time of pulling up a silicon single crystal ingot.
The silicon single crystals fabricated by CZ method also contain a crystalline defect, a ring-like OSF (Oxidation induced Stacking Fault). The OSF causes malfunctions in substrates of semiconductor devices, such as increase in leak of electric current, and moreover impairs solar cell properties of solar cell substrates. It is known that the OSF in the silicon single crystals is inhibited by carbon doping at the time of pulling up a silicon single crystal ingot.
It is known that silicon single crystals for the provision of substrates of semiconductor devices and solar cell substrates, if no impurities are contained therein, have a reduced strength and undergo dislocation due to thermal stress in the second half of the pulling-up operation. Approaches to prevent the dislocation from occurring are allowing oxygen to be actively incorporated from a quartz crucible and the doping of impurities such as carbon and nitrogen thereby enhancing the strength of silicon single crystals. In particular, incorporating carbon at a small amount ranging from about 0.01 to 1 ppma (at a state of silicon melt) is effective, because this method would highly increase the strength of silicon single crystals without impairing electric properties of the silicon single crystals. Meanwhile, in solar cell substrates, the demand for reducing the cost of solar photovoltaic generation calls for silicon single crystal producible at a high yield with lower cost.
Recent approach in view of the above is the fabrication of silicon single crystals intentionally doped with carbon. Proposed methods for doping crystals with carbon involve the use of carbon powder (Patent Literature 1) and the use of solid carbon (Patent Literature 2). Those doping methods with the use of solid carbon still have problems such as letting carbon not mixed or dissolved float in a silicon melt thereby making the single crystals remain liable to have dislocations, and the inability to dope carbon at a necessary concentration with favorable precision. Another disadvantage was that as a result of silicon convection with an extremely high concentration of carbon at the bottom of the crucible at an initial stage of the silicon raw material being dissolved within the crucible, the inner wall of the quartz crucible reacts with carbon and this shortens the durability of the quartz crucible.
A method proposed to address the above problems is described in Patent Literature 3. In this document, the inclusion of carbon at a necessary concentration at a favorable precision is done by using, as part of a polycrystalline silicon raw material, a polycrystalline silicon containing carbon at a high concentration of 3 ppma or higher. This method provides the polycrystalline silicon containing carbon at 3 ppma or higher in the form of a thin plate, and subjects this thin plate to acid etching before its use. And yet, no more particular mention is made in this document on the availability of that polycrystalline silicon. The fact is that the attempt to obtain the polycrystalline silicon having a high carbon concentration in the form of a thin plate would require special production process, which will increase the production cost, and involve the concern for possible contamination from other metals than carbon.
Also, the polycrystalline silicon in the shape of the thin plate is inferior in its flowability and in its operability at the time of its introduction into a crucible. Particularly in the production of the silicon single crystals by a process including pulling up a single silicon single crystal, allowing a silicon raw material to be recharged into the crucible and dissolved, and thereafter pulling up a silicon single crystal again, namely a multi-pulling process, the polycrystalline silicon in the shape of the thin plate is inapplicable as the silicon raw material to be recharged, due to its inability to flow downward smoothly within a recharging quartz tube.
Furthermore, the inclusion in the polycrystalline silicon of carbon at a high concentration of 3 ppma or higher can involve the failure for the carbon to be uniformly mixed into the silicon melt thereby localizing the regions with high carbon concentration, possibly causing the single crystals to have dislocation.